The ink jet printing system has been used as one of the printing systems. This ink jet printing system is a recording system, which comprises the step of printing by discharging ink drops through a discharge orifice such as a nozzle or a slit to thus make the ink drops directly adhere to a printing member and can usually fall into two broad categories: continuous injection systems and on-demand systems. The ink mainly used in such an ink jet printing system comprises a dye dissolved in an aqueous or nonaqueous solvent.
On the other hand, in the fields of colored paints and conductive paints, a liquid dispersion of metal ultrafine particles has been used, but there has not yet been used such a liquid dispersion of metal ultrafine particles in the formation of a conductive circuit, while making use of the ink jet printing system. Such a conductive circuit has conventionally been formed according to the screen printing techniques or a technique, which makes use of a photoresist and in this case, a large-scaled apparatus should be used and complicated processes should likewise be used.
As methods for preparing the foregoing liquid dispersion of metal ultrafine particles, there have been known those comprising the step of dispersing metal ultrafine particles or powder together with, for instance, a solvent, a resin and a dispersant, according to various means such as stirring, the applying of ultrasonic waves and mixing in, for instance, a ball mill or a sand mill. The liquid dispersion prepared according to this method has been employed in the fields of, for instance, paints and varnishes. There have been known, for instance, a method for directly preparing metal ultrafine particles in a liquid phase such as the technique for the evaporation of a metal in a gas phase (hereunder referred to as “evaporation-in-gas (technique)” comprising the steps of evaporating a metal in a low vacuum atmosphere in the coexistence of vapor of a solvent, and then condensing the evaporated metal into uniform metal ultrafine particles to thus disperse the resulting particles in the condensed solvent and to thus give a liquid dispersion (Japanese Patent No. 2,561,537) and those, which make use of an insoluble precipitate-forming reaction or a reducing reaction using a reducing agent. Among these methods for preparing liquid dispersions of metal ultrafine particles, the evaporation-in-gas technique would permit the stable preparation of a liquid dispersion containing metal ultrafine particles having a particle size of not more than 100 nm and which are uniformly dispersed therein. In the evaporation-in-gas technique, the amount of a dispersion stabilizer or a resin component required for the preparation of a liquid dispersion containing metal ultrafine particles in a desired concentration is smaller than that required in the liquid-phase preparation technique.
As has been described above, the liquid dispersion of metal ultrafine particles has never been used as the ink for ink jet printer or recording (system) for the reason that the conventional liquid dispersion of metal ultrafine particles has never possessed characteristic properties (such as viscosity and surface tension) required for the ink used in the ink jet printing. The metal ultrafine particles prepared according to the conventional evaporation-in-gas technique undergo aggregation and this makes it difficult to prepare any liquid dispersion in which the metal ultrafine particles are stably dispersed. For this reason, when using such a liquid dispersion of metal ultrafine particles as ink for the ink jet printing, the ink suffers from a problem in that aggregates of the ultrafine particles present therein result in clogging of the ink jet nozzles. Moreover, when using a liquid dispersion in which the ultrafine particles are independently or separately dispersed, as ink for the ink jet printing, the liquid dispersion should be prepared using a solvent suitable for satisfying the requirements for characteristic properties of the ink. However, the choice of a solvent suitably used in the ink for the ink jet printing has been quite difficult.
In addition, in the conventional evaporation-in-gas technique, the solvent used is denatured when the evaporated metal is condensed to thus form by-products and accordingly, the resulting liquid dispersion would suffer from various problems such as storage stability, viscosity stability and coloring, depending on the amount of the by-products. Furthermore, in some cases or in some applications, there is a request in that low boiling point solvents, water and alcohol-containing (alcoholic) solvents, which can be used in the evaporation-in-gas technique with great difficulty, must be used for the preparation of liquid dispersions of metal ultrafine particles, as will be detailed below.
Accordingly, it is an object of the present invention to eliminate the foregoing problems associated with the conventional techniques and to provide ink, which comprises a liquid dispersion containing individually dispersed metal ultrafine particles and which can be used in the ink jet printing technique or which can satisfy the requirements for the ink jet printing ink as well as a method for preparing the same.